Method for producing cemented carbide or cermet products

ABSTRACT

The present invention relates to a method for the production of tungsten carbide based cemented carbide or cermet tools or components using the powder injection moulding or extrusion method comprising mixing of granulated cemented carbide or cermet powder with an organic binder system whereby the mixing is made by adding all the constituents into a mixer heated to a temperature above the melting temperature of the organic binders. According to the invention the organic binders are added into the mixer, waiting for a melt to form and then slowly adding the cemented carbide or cermet powder into the melt, making sure the temperature does not fall below the melting temperatures of the organic binders.

The present invention relates to a method for the production of tungsten carbide based or cermet hard metal tools or components using the powder injection moulding or extrusion method and a method for producing a binder system therefore.

Hard metals based on tungsten carbide are composites consisting of small (μm-scale) grains of at least one hard phase in a binder phase. These materials always contain the hard phase tungsten carbide (WC). In addition, other metal carbides with the general composition (Ti, Nb, Ta, W) C may also be included, as well as metal carbonitrides, e.g., Ti (C, N). The binder phase usually consists of cobalt (Co). Other binder phase compositions may also be used, e.g., combinations of Co, Ni, and Fe, or Ni and Fe.

Industrial production of tungsten carbide based hard metals often includes blending of given proportions of powders of raw materials and additives in the wet state using a milling liquid. This liquid is often an alcohol, e.g. ethanol or water, or a mixture thereof. The mixture is then milled into a homogeneous slurry. The wet milling operation is made with the purpose of deagglomerating and mixing the raw materials intimately. Individual raw material grains are also disintegrated to some extent. The obtained slurry is then dried and granulated, e.g. by means of a spray dryer. The granulate thus obtained may then be used in uniaxial pressing of green bodies or for extrusion or injection moulding.

Injection moulding is common in the plastics industry, where material containing thermoplastics or thermosetting polymers are heated and forced into a mould with the desired shape. The method is often referred to as Powder Injection Moulding (PIM) when used in powder technology. The method is preferably used for parts with complex geometry.

In powder injection moulding of tungsten carbide based hard metal parts, four consecutive steps are applied:

1. Mixing of the granulated cemented carbide powder with a binder system. The binder system acts as a carrier for the powder and constitutes 25-60 volume % of the resulting material, often referred to as the feedstock. The exact concentration is dependent on the desired process properties during moulding. The mixing is made by adding all the constituents into a mixer heated to a temperature above the melting temperature of the organic binders. The resulting feedstock is obtained as pellets of approximate size 4×4 mm.

2. Injection moulding is performed using the mixed feedstock. The material is heated to 100-240° C. and then forced into a cavity with the desired shape. The thus obtained part is cooled and then removed from the cavity.

3. Removing the binder from the obtained part. The removal can be obtained by extraction of the parts in a suitable solvent and/or by heating in a furnace with a suitable atmosphere. This step is often referred to as the debinding step.

4. Sintering of the parts. Common sintering procedures for cemented carbides are applied.

Extrusion of the feedstock comprises steps 1, 3 and 4 above. Instead of forcing the feedstock into a cavity of the desired shape, the feedstock is continuously forced through a die with the desired cross section.

The solids loading, φ, of the feedstock is the volumetric amount of hard constituents, compared to the organic constituents. φ can be calculated using the following equation:

$\varphi = \frac{\rho_{f} - \rho_{v}}{\rho_{s} - \rho_{v}}$

where ρ_(S) is the density of the cemented carbide as sintered, ρ_(ν) is the mean density of the organic constituents and ρ_(ƒ) is the density of the feedstock, measured with a helium pycnometer.

When mixing the hard constituents with the organic binders, it is a common problem that a part of the organic binder does not spread properly in the feedstock. Instead, a small part of the organic binder forms particles, considerably larger than the grain size of the hard constituents, i.e. in the range of 10-30 •m. During the debinding of the green body, these particles will be removed, leaving pores in the structure. A common way to remove these pores is to use sintering with applied hydrostatic pressure of Ar, i.e., Sinter-HIP:ing. When using sinter-HIP:ing, the pores will be filled with the metallic binder phase if the pores have no physical connection with the applied pressure. Pores close to the surface of the green body will instead collapse to form surface pores, as will pores located directly in the surface of the green body. The pores in the surface will severely decrease the macroscopic mechanical strength of the sintered material. The metallic binder filled former pores in the bulk of the material will decrease the mechanical strength of the sintered material as well. Another common problem in case of the particles of organic binders being large, i.e. in the range of 20-30 •m, these particles will pyrolyse with a too fast development of gases during the debinding step, forming blisters in the material structure.

It is an object of the present invention to solve these problems.

FIG. 1 shows a LOM, light optical micrograph, with a magnification about 1000x of the microstructure of a cemented carbide according to prior art.

FIG. 2 shows a LOM, light optical micrograph, with a magnification about 1000x of the microstructure of a cemented carbide according to the invention.

It has now surprisingly been found that by adding the organic binders into the mixer, waiting for a melt to form and then slowly adding the hard constituents into the melt, making sure the temperature does not fall below the melting temperatures of the organic binders, no organic binder particles are formed and the abovementioned problems can be solved.

The method according to the present invention comprises the following steps:

1) Wet milling of the raw materials in water or alcohol, or a combination thereof, preferably 80 wt-% ethanol and 20 wt-% water, together with 0.1-1.2 wt-%, preferably 0.25-0.55 wt-% carboxylic acid, preferably stearic acid as a granulating agent for the subsequent drying. More carboxylic acid is required the smaller the grain size of the hard constituents.

2) Drying of the slurry formed during the above mentioned wet milling process step.

3) Mixing the dried powder by kneading with a binder system, consisting of 30-60 wt-% olefinic polymers, 40-70 wt-% waxes and to a solids loading of φ=0.54-0.56. The mixing is performed in a batch mixer or a twin screw extruder. When using a batch mixer, the organic binders are added first to the heated mixer. The polymers are added first and then the waxes. When a melt is formed, the powdered hard constituents are slowly added, making sure the temperature does not fall below the melting temperatures of the organic binders. When a twin screw extruder is used for the mixing, the organic binders are added in the beginning of the screw and the powdered hard constituents are added by side feeders, making sure the powders are mixed into a melt and also making sure the temperature does not fall below the melting temperature of the organic binders. The powdered hard constituents can be added through several side feeders along the twin screw extruder or the material can be run through the twin screw extruder several times to make sure the temperature does not fall below the melting temperature of the organic binders. Alternatively, the powdered hard constituents are pre heated before being added to the molten organic binder to make sure that the temperature does not fall below the melting temperature of the organic binders. The material is then formed into pellets with a size of approximately 4×4 mm.

4) Injection moulding of the feedstock in a conventional injection moulding machine. Alternatively, the feedstock is extruded in a single screw, twin screw or plunge type extruder. The material is heated to 100-240° C., preferably 140-160° C., and then, in the case of injection moulding, forced into a cavity with the desired shape. In extrusion, the material is forced through a die with the desired cross section. The part obtained in injection moulding is cooled and then removed from the cavity. The extrudates are cut in pieces of desired length.

5) Debinding the obtained part. The debinding is performed in two steps.

5a) By extraction of the wax and petroleum jelly in an apolar solvent, at 31-70° C., preferably at 31- 55° C. It is within the purview of the skilled artisan to determine by experiments the conditions necessary to avoid the formation of cracks and other defects according to this specification.

5b) By heating in a furnace, preferably in a flowing gaseous medium atmosphere at 2 mbar to atmospheric pressure up to 450° C. It is within the purview of the skilled artisan to determine by experiments the conditions necessary to avoid the formation of cracks and other defects according to this specification.

6) Presintering of the part in the debinding furnace in vacuum at 900-1250° C., preferably at about 1200° C.

7) Sintering of the parts using conventional sintering technique.

The invention can be used for all compositions of cemented carbide and all WC grain sizes commonly used as well as for titanium carbonitride based materials.

In one embodiment the WC or Ti (C, N) grain size shall be 0.2-1.5 μm with conventional grain growth inhibitors.

In another embodiment the WC or Ti (C, N) grain size shall be 1.5-4 μm.

Example 1

A WC-13 wt-% Co submicron cemented carbide powder was made by wet milling 780 g Co-powder (OMG extra fine), 38.66 g Cr₃C₂ (H C Starck), 5161 g WC (H C Starck DS80), 20.44 g W metal powder, 16 g Fisher-Tropsch wax (Sasol H1) and 22 g stearic acid in 1.6 1 milling liquid consisting of ethanol and water (80:20 by weight) for 40 h. The stearic acid is added in this stage of the process to work as a granule forming agent, when spray drying the slurry. The resulting slurry was spraydried to a granulated powder.

Example 2 (Comparative)

The powder made in Example 1 was mixed by kneading 2500 g powder from Example 1 with 50.97 g Polypropylene-polyethylene copolymer (RD360 MO, Borealis) and 50.97 g Paraffin wax (Sasol Wax) in a Z-blade kneader mixer (Werner & Pfleiderer LUK 1, 0). The Z-blade kneader was heated to 170° C and the raw material was added. The mixer was run until a smooth viscous feedstock developed. This resulted in a feedstock with a density of 8.23 g/ml, corresponding to a φ of 0.553.

Example 3 (Invention)

The powder made in Example 1 was mixed by kneading 2500 g powder from Example 1 with 50.97 g Polypropylene-polyethylene copolymer (RD360 MO, Borealis) and 45.87 g Paraffin wax (Sasol Wax) and 5.06 g petroleum jelly (Merkur VARA AB) in a Z-blade kneader mixer (Werner & Pfleiderer LUK 1, 0). The Z-blade kneader was heated to 170° C and the organic binders were added to the mixer. The polymer was added first and then the waxes. When a melt was formed, the powdered hard constituents were slowly added, making sure the temperature did not fall below the melting temperatures of the organic binders. The mixer was run until a smooth viscous feedstock developed. This resulted in a feedstock with a density of 8.23 g/ml, corresponding to a φ of 0.553.

Example 4 (Comparative)

The feedstock made in example 2 was fed into an injection moulding machine (Battenfeld HM 60/130/22). The machine was used for the injection moulding of a Seco Tools Minimaster 10 mm endmill green body.

Example 5 (Invention)

The feedstock made in example 3 was fed into an injection moulding machine (Battenfeld HM 60/130/22). The machine was used for the injection moulding of a Seco Tools Minimaster 10 mm endmill green body.

Example 6 (Comparative)

The parts from example 4 were debound by extraction and sintered in a Sinter-HIP furnace (PVA COD733R) at 1420° C. with a total soaking time of 60 min. After 30 min at the peak hold temperature, the furnace pressure was raised to 3 MPa Ar.

After sintering, the parts were cut for inspection. The parts from example 4 were free from carbon pores, eta-phase and pores, i.e. A00 B00 C00 according to ISO 4505. The parts showed Co-lakes and open surface pores. See FIG. 1.

Example 7 (Invention)

The parts from example 5 were debound by extraction and sintered in a Sinter-HIP furnace (PVA COD733R) at 1420° C. with a total soaking time of 60 min. After 30 min at the peak hold temperature, the furnace pressure was raised to 3 MPa Ar.

After sintering, the parts were cut for inspection. The parts from example 5 were free from carbon pores, cracks, eta-phase and pores, i.e. A00 B00 C00 according to ISO 4505. There were no surface pores and the microstructure showed an even cobalt distribution. See FIG. 2. 

1. Method for the production of tungsten carbide based cemented carbide or cermet tools or components using the powder injection moulding or extrusion method comprising mixing of granulated cemented carbide or cermet powder with an organic binder system acting as a carrier for the powder whereby the mixing is made by adding all the constituents into a mixer heated to a temperature above the melting temperature of the organic binders characterised in that the organic binders are added into the mixer, waiting for a melt to form and then slowly adding the cemented carbide or cermet powder into the melt, making sure the temperature does not fall below the melting temperatures of the organic binders.
 2. Method according to claim 1 characterised in that the mixing is performed in a batch mixer.
 3. Method according to claim 1 characterised in that the mixing is performed in an extruder.
 4. Method according to claim 3 characterised in that the extruder is a twin screw extruder. 